The invention relates to a fuel control system for an internal combustion engine having an exhaust conduit, comprising:
a probe in the exhaust conduit adapted to generate an electrical signal indicative of exhaust conditions, PA1 at least one circuit for supplying fuel and air to the engine, having solenoid valve means for metering at least the fuel flow rate in said circuit, PA1 a closed loop electronic control circuit connected to said probe and effective to control said solenoid valve means in dependence of said signal, PA1 and means for opening the loop responsive to predetermined operating conditions of the engine. PA1 when the engine temperature is below a first predetermined value, PA1 during acceleration and under full load when the engine temperature is between the first value and a second predetermined value, and/or PA1 under full load when the engine temperature is above the second predetermined value.
An engine cannot operate satisfactorily on a stoichiometric fuel-air mixture under all conditions. A stoichiometric mixture is satisfactory when the engine is at its normal operating temperature and runs at a constant speed under moderate load; on the other hand the richness must be increased under certain operating conditions, e.g. when the engine runs while cold after starting, when the engine is under full load, or during acceleration.
The enrichment may be obtained by opening the regulation loop so that the device operates in the same manner as a conventional carburation device. However, this approach is far from being fully satisfactory. Upon transition from closed loop to open-loop operation, there is instantaneous loss of the automatic adjustment which is inherent to closed-loop operation. As is known, the optimum richness of the mixture depends on operating parameters which then prevail, inter alia on the engine temperature but also, to a lesser extent, on external factors such as the ambient temperature and atmospheric pressure.
It is an object of the invention to provide a system of the above defined kind wherein the disadvantages resulting from the transition from closed-loop to open-loop operation are at least partially overcome.
According to a first aspect of the invention, the electronic circuit has further means for controlling said solenoid valve means in dependence on at least one further engine operating parameter during open loop operation.
According to another aspect, the electronic circuit has memory means for storing a value representative of the adjustment of the solenoid valve means during closed-loop operation and means for controlling the solenoid valve by adjusting it from the stored value upon opening of said loop.
An acceptable compromise between the somewhat contradictory requirements of easy driving and minimum pollution by the exhaust gases can be achieved with such arrangements.
The electronic circuit is typically of a kind wherein the solenoid valve is controlled by supplying it with periodic rectangular electric signals with a cyclic or aperture ratio which determines the average time during which the solenoid valve is opened during a given time interval. Then upon transition to open-loop operation, enrichment can be brought about by modifying the aperture ratio, starting from the stored value, in accordance with a function depending on an engine operating parameter such as its temperature.
The electronic circuit is e.g. adapted for open-loop operation under the following conditions:
When the electronic circuit comprises "volatile" memory means, the content of which is last if the supply is cut off, the problem arises of restarting the engine after a stop. This problem can be solved in a number of ways. The memory means can have a separate power supply which remains available even when the engine is stopped. Another solution is to provide the memory means with auxiliary means which sets a reference value (either fixed or adjustable by the driver) for starting under open-loop conditions.
It will generally be advantageous to use on/off valves, in which case the flow rate of fuel supplied to the engine is adjusted by modifying the cyclic or aperture ratio RCO.
Under these conditions, adjustment during transition from closed-loop to open-loop operation (which will always correspond to an increase in the aperture ratio of the solenoid valve) can be made either by increasing the aperture ratio by an amount depending on the engine operating parameter, or by multiplying it by a similarly dependent factor.
During closed-loop operation, the adjustment or setting can be stored to correspond to a predetermined engine load.
Jerky operation can be avoided if the closed-loop regulating circuit is automatically and permanently adjusted to the open-loop regulating circuit when the latter controls operation of the vehicle.
In particular embodiments, the system can comprise a single solenoid valve placed in the main fuel supply circuit (possibly in parallel flow with a permanent flow calibrated orifice). Usually, however, it is preferable to provide at least a second solenoid valve which is actuated simultaneously with the first valve and is placed in the engine idling circuit.
Other valves can be added, inter alia a valve for supplying additional air to the engine.
Currently available solenoid valves are open when not energized. Advantageously, to prevent operation resulting from auto-ignition after the contact has been cut off, a quenching circuit is provided so as to keep the solenoid valves closed for a predetermined time after the contact has been cut.
The invention will be better understood from the following description of carburation systems constituting particular embodiments thereof. The description refers to the accompanying drawings.